Trans-Tethyan correlation of the Lower–Middle Cenomanian boundary interval; southern England (Southerham, near Lewes, Sussex) and Douar el Khiana, northeastern Algeria

A 480 m section of marls with widely separated levels of nodular limestone in the Fahdene Formation north of Bou Khadra in Tebessa Province, northeastern Algeria, spans the Lower/Middle Cenomanian boundary. A total of 30 ammonite species are present, of which two: Forbesiceras reversum and Calycoceras (Newboldiceras) algeriense are new. The fauna allows recognition of the Northwest European upper Lower Cenomanian Mantelliceras dixoni Zone, the succeeding lower Middle Cenomanian Cunningtoniceras inerme Zone, the Acanthoceras rhotomagense Zone and its subzones of Turrilites costatus and Turrilites acutus. The sequence of index species occurs in the same order in both north-eastern Tunisia and the Southerham Grey Pit in Sussex (and indeed elsewhere in Northwest Europe), indicating these to be robust assemblage zones and subzones that can be recognised on both the north and south sides of the Tethys. Other occurrences of taxa that are common in both sections and regions are markedly different, and include the co-occurrence of Cunningtoniceras inerme (Pervinquière, 1907) with Acanthoceras rhotomagense (Brongniart, 1822) in the costatus Subzone in north-eastern Algeria and central Tunisia, the extension of Acompsoceras renevieri (Sharpe, 1857) into the lower Middle Cenomanian in north-eastern Tunisia, whilst the acme of Turrilites scheuchzerianus Bosc, 1801, is in the dixoni Zone in Northwest Europe, and in the inerme Zone in northeasten Algeria and adjacent parts of Central Tunisia. These differences are not a result of collection failure or non-preservation, but must rather reflect environmental controls on occurrence and abundance.